1. Field of the Invention
The present invention relates to a hydraulic excavator.
2. Background Information
A hydraulic excavator is provided with working implement including a boom. The boom is driven by a hydraulic cylinder. The hydraulic cylinder is driven by hydraulic fluid discharged from a hydraulic pump. The hydraulic excavator is provided with a revolving upper unit and an undercarriage. The revolving upper unit rotates by being driven by a hydraulic motor. The hydraulic motor is driven by hydraulic fluid discharged from the hydraulic pump. For example, the hydraulic cylinder and the hydraulic motor in a conventional hydraulic excavator disclosed in Japanese Laid-open Patent Publication No. 2003-004005 are connected in parallel to the hydraulic pump.
Boom raising and rotating operations are performed at the same time in the hydraulic excavator. For example, the hydraulic excavator rotates while raising the boom to place excavated dirt beside the vehicle when performing work to excavate a ditch. Alternatively, the hydraulic excavator rotates while raising the boom to deposit dirt in a dump truck parked beside the hydraulic excavator when performing loading work. When these boom raising and rotating operations are conducted at the same time, the relationship between a time needed for moving the revolving upper unit to a certain position and an amount of raising the boom during the time needs to be adjusted carefully. For example, when performing loading work onto a dump truck, the boom needs to be raised so that the bucket reaches a position higher than the bed of the dump truck when the revolving upper unit is rotating to the position of the dump truck.
The hydraulic cylinder and the hydraulic motor in the abovementioned conventional hydraulic excavator are connected in parallel to a hydraulic pump. As a result, a rotation motor is driven with hydraulic pressure that is the same as the hydraulic pressure for driving the boom cylinder when the boom raising and the rotating operations are conducted at the same time. Thus, the rotating time of the revolving upper unit and the boom raising amount during the rotating time can be synchronized to achieve a certain relationship.
Conversely, a hydraulic excavator has recently been proposed that is provided with a closed hydraulic pressure circuit for supplying hydraulic fluid to the hydraulic cylinder as described in Japanese National Publication of International Patent Application No. 2009-511831 Potential energy of the working implement is regenerated due to the hydraulic circuit being a closed circuit. As a result, the fuel consumption of the motor for driving the hydraulic pump can be reduced.